Four-post magnet MRI systems are disclosed in U.S. Pat. No. 4,829,252 and advantageously provide improved access to the patient and a less enclosed environment for the patient during the MRI process.
A transmission coil that may be used in conjunction with the four-post MRI transverse magnet system to retain the open environment within the imaging volume, while reducing the power requirements for the transmission coil is disclosed in U.S. application No. 08/285,008. This transmission coil is sometimes referred to as the "X-wing coil."The disclosures of U.S. Pat. No. 4,829,252 and U.S. application No. 08/285,008, referred to above, and describing the four-post MRI system and the X-wing transmission coil, are incorporated herein by reference.
The four-post MRI system and the X-wing coil can be seen, for example, in portions of FIG. 1. The four-post MRI assembly 10 includes posts 1-4 supporting an upper transverse magnet assembly 20 and a lower transverse assembly 22. Also included are magnet pole pieces 24 and 26 and pancake-like gradient coil assemblies 23A and 23B, respectively at the upper and lower portions of the imaging volume 25. The X-wing transmission coil 12 is near the posts 1-4 and the magnet pole pieces 24 and 26.
To maintain a high quality factor in the X-wing transmission coil, the X-wing coil should not be near any similarly shaped object outside of the image volume 25. Unfortunately, in the four-post system, the four-posts 1-4, which are close to the X-wing coils, provide a similar shape to the X-wing coils themselves. This results in inductive coupling between the X-wing transmission coil 12 and the posts 1-4. The posts 1-4 are magnetically permeable material and are RF "absorbers" during the transmission cycle of the X-wing coil, causing the X-wing coil to be lossy and specifically causing the coil Q to be substantially reduced.
The quality factor Q, that is associated with the transmission coil, is a well-known characteristic that can be described mathematically as: ##EQU1## Also, the RF transmission power requirements, P, advantageously decrease with increases in coil Q according to: ##EQU2## Thus, a high Q in the transmission coil will provide decreases in the required RF transmission power.
Unfortunately, the X-wing transmission coil, when used in conjunction with the four-post MRI system, can result in low coil Q values. Specifically, with today's technology, transmission coil Q's of 300 or more for high field MRI systems are desirable. For low field MRI systems, transmission coil Q's of 110 and higher are desirable. An unprotected X-wing transmission coil in the four-post MRI, however, can result in substantially lower Q values.
Ideally, a four-post MRI system should use the X-wing coil to gain improvements in access to the patient, yet still obtain reductions in transmission power requirements, and improvements in the transmission coil Q.